The manufacture of a pivot arbor for a timepiece consists in performing bar turning operations on a hardenable steel bar to define various active surfaces (bearing surface, shoulder, pivots, etc.) and then in subjecting the bar-turned arbor to heat treatments comprising at least one hardening operation to improve the hardness of the arbor and one or more tempering operations to improve its tenacity. The heat treatment operations are followed by an operation of rolling the pivots of the arbors, which consists in polishing the pivots to the required dimensions. The hardness and roughness of the pivots are further improved during the rolling operation.
The pivot arbors, for example the balance staffs, conventionally used in mechanical timepiece movements are made of steel grades for bar turning which are generally martensitic carbon steels containing lead and manganese sulphides to improve their machinability. A known steel of this type, named 20AP, is typically used for these applications.
This type of material has the advantage of being easy to machine, in particular of being suitable for bar turning and, after hardening and tempering, has superior mechanical properties which are very advantageous for making timepiece pivot arbors. These steels have a high hardness, making it possible to obtain very good shock resistance, particularly after heat treatment. Typically, the hardness of arbor pivots made of 20AP steel can exceed 700 HV after heat treatment and rolling.
Although this type of material provides satisfactory mechanical properties for the horological applications described above, it has the drawback of being magnetic and capable of interfering with the working of a watch after being subjected to a magnetic field, particularly when the material is used to make a balance staff cooperating with a balance spring made of ferromagnetic material. This phenomenon is well known to those skilled in the art. It will also be noted that these martensitic steels are also sensitive to corrosion.
Attempts have been made to try to overcome these drawbacks with austenitic stainless steels, which have the peculiarity of being non-magnetic, namely paramagnetic or diamagnetic or antiferromagnetic. However, these austenitic steels have a crystallographic structure, which does not allow them to be hardened and to achieve levels of hardness and thus shock resistance compatible with the requirements necessary for making timepiece pivot arbors. The arbors obtained then exhibit marks or severe damage in the event of shocks, which will then have a negative effect on the chronometry of the movement. One means of increasing the hardness of these steels is cold working, however this hardening operation cannot achieve hardnesses of more than 500 HV. Consequently, for parts requiring pivots with a high shock resistance, the use of this type of steel remains limited.
Another approach for attempting to overcome these drawbacks consists in depositing hard layers of materials such as diamond-like-carbon (DLC) on the pivot arbors. However, there have been observed significant risks of delamination of the hard layer and thus the formation of debris which can move around inside the timepiece movement and disrupt the operation of the latter, which is unsatisfactory.
There are also known, from EP 2 757 423, pivot arbors made of an austenitic alloy of cobalt or nickel and having an outer surface hardened to a certain depth. However, such alloys may prove difficult to machine for the manufacture of pivot arbors. Moreover, they are relatively expensive because of the high cost of nickel and cobalt.